1. Field of the Invention
This invention relates, in general, to electrical bushings for transformers and, more specifically, to bushings containing current-limiting fuses.
2. Description of the Prior Art
A wide variety of bushing containing current-limiting fuses have been used in conjunction with electrical apparatus, such as distribution transformers, according to the prior art. In order to satisfactorily protect the transformer and the associated electrical distribution system, it has been usual practice to place a protective link in series with the current-limiting fuse disposed within the bushing assembly. This has been required since the current interrupting characteristics of most prior art current-limiting fuses have been such that only large fault currents could be interrupted by the current-limiting fuse. To provide a full range of primary current protection, it was necessary to install the protective link in series with the current-limiting fuse to interrupt the primary circuit upon the initiation of a fault which causes a current which is too small for interruption by the current-limiting fuse.
Generally, such arrangements have the current-limiting fuse positioned at the upper end of the bushing insulator for the purpose of keeping the current-limiting fuse out of the liquid dielectric of the transformer. This has been required in the prior art because operation of conventional current-limiting fuses disposed directly in the transformer dielectric was not satisfactory. The protective link generally was connected to the lower end of the current-limiting fuse and extended somewhat form the bushing insulator. Due to the construction of the protective link, it has been possible to provide satisfactory performance with the protective link immersed in the liquid dielectric of the transformer.
With the development of a full-range current-limiting fuse which interrupts current from relatively low overload values to extremely high fault current values, it has been possible to provide a fused bushing assembly which does not require a protective link in series with a limited or partial range current-limiting fuse. Thus, the structural supporting arrangements herebefore used with current-limiting fuses and protective links disposed within the bushing insulator are not applicable to a bushing constructed for use with a full-range current-limiting fuse. Therefore, it is desirable, and it is an object of this invention, to provide an arrangement for mounting and securing a full-range current-limiting fuse within the insulating structure of an electrical bushing which is suitable for mounting on the enclosure of a distribution transformer.